When giving a presentation, there are some cases in which only an image field of a specific window rather than the entire screen displayed on a display device needs to be displayed on an external monitor. There is known a technology of outputting to an external monitor only an image of the window selected with a mouse pointer among a plurality of windows displayed on a display device (see, e.g., Japanese Patent Publication No. 2000-339130).
However, consideration is not made of connecting together a plurality of image supply devices with an image display device and improving the operability in such case.